The following terminology is adopted in this disclosure.
Cell: The Cell 10 as described in FIG. 1 is the most basic element of a battery system, with positive and negative terminals, storing and dispensing electrical energy through an electrochemical process. For example, it could be a nominal 3.7 V Lithium ion cylindrical cell or a nominal 2.1V Lead-Acid prismatic cell. A Cell is usually characterized by its AC Impedance (ACI), Equivalent Series Resistance (ESR), Capacity (in Amp.Hour, or in short, Ah), and Nominal Cell Voltage. The manufacturer typically provides many other parameters, such as cycle life, optimal temperature, maximum charge and discharge rate.
Block: The Block 20, as described in FIG. 2, is a collection of Cells 10 wired directly in parallel, providing the same voltage as individual Cells. All the Cells in a block must belong to the same chemistry. For instance, all Lithium Carbonate (LCO) cells, or all Lead-Acid cells. A typical Block may have as few as 1 Cell and some times as many as 1000 Cells or more. The current collector 21 is a conductive path, typically a metallic plate that is connected to all the positive terminals of the Cells in the Block. There are many methods of connection including soldering, welding, and spring contact. The current collector 22 is a conductive path, typically a metallic plate that is connected to all the negative terminals of the Cells in the Block. Methods of connection are similar to that of the positive side.
Although a stacked approach is shown in FIG. 2 for building a Block out of Cells, there are many other ways of making a Block as known to an expert in the field of battery manufacturing. For instance, many cells may be inserted into a set of spring-contact connectors, and the respective conductive contacts may then be electrically joined together to make a Block.
While these examples are cited for the reason of comprehension, it is to be understood that a Block is essentially a collection of Cells connected electrically in parallel.
Battery: A collection of Blocks wired in series. For instance, a 3S4P Lead-Acid battery consists of 3 Blocks wired in Series, with each Block containing 4 Cells in parallel. Such a battery would have a nominal voltage of 6.3V (Three in series multiplied by 2.1V of nominal Cell voltage). In FIG. 3 an example of a 3S4P Battery is shown. The battery 30 consists of three Blocks—35, 36 and 37. The positive terminal of the first battery 35 is typically connected to a current carrying wire 31, and is available to external devices as the positive terminal for the entire battery. The negative terminal of the last battery 37 is typically connected to a current carrying conductor 32, and is available to external devices as the negative terminal for the entire battery.
The Series connection is realized by connecting opposite parity terminals of consecutive blocks. For instance, in FIG. 3, the negative plate 22 of the top block 35 is connected electrically to the positive terminal 21 of the middle block 36.
The Blocks connected such may be enclosed in a mechanical cover 33 for safety or mechanical convenience.
In certain instances a Battery may be packaged in such a way that Cells of the same Block may be placed at different mechanical locations, but electrically they would be considered to belong to the same Block. In FIG. 4 we show an example of this, wherein the Battery 40 consists of two mechanical assemblies 44 and 45. It is to be noted that the two assemblies are indeed connected in parallel at the Block level. For example, the group of Cells 46 are electrically connected in parallel with the group of Cells 47. The same applies to other groups of Cells. In this case, the groups of Cells 46 and 47 belong to a single Block. For the purposes of this disclosure, this Battery would be considered as 3S8P, consisting of 3 Blocks, with each Block containing 8 Cells. The groups 46 and 47 for instance, form one Block of 8 Cells.
Battery Management System (BMS): An electronic system that has components addressing, monitoring and communicating between Blocks to control the electron flow to create a balance between all the Blocks according to a pre-determined logic. The BMS also makes decisions, such as disengaging the Battery from the outside electricals in the event of high voltage, high charge or discharge current, high internal or external temperatures, Cell failures, and re-engaging when such conditions are rectified.
Pack: A Battery mechanically and electrically packed with a Battery Management System (BMS) voltage, current, and thermal sensors, and optionally active or passive thermal control devices to keep the battery at a desired temperature range.
FIG. 5 shows a Battery with 3 Blocks in series. The Battery may have been charged and discharged through any number of cycles. If voltages of all the Blocks are identical or nearly identical (typically within +/−3%), then the Battery is considered to be balanced. In the case of FIG. 5, all the three Blocks have 4.2V across them—hence the Battery is balanced. In FIG. 6, the Battery has 3 Blocks, but at a given instant of time, the voltages across the Blocks are 4.4V, 4.0V and 4.2V—all different significantly from one another. (at least one Block >3% off from at least one other Block). Such a Battery is called unbalanced.
In FIG. 7 we show a BMS that exists in the prior art and is commercially available. The Battery 30 is connected to a charger 51 and a load 52 at its positive terminal. A BMS 55 is connected to the Battery in a way that it has electrical access to every terminal of every Block. For instance, the electrical line 56 is connected to the connection wire 34 between the top and the middle Block.
The electrical circuit from the charger or the load goes through the Battery positive and negative terminals, and is terminated back through the BMS. The negative terminal connections are not shown to maintain the clarity of the figure. A practicing engineer in the field will know that the negative terminals of the Battery, the Charger, the Load and the BMS would be tied together. The BMS therefore has the capability to close or open the electrical circuits for charging or discharging (through the load) upon certain conditions. In FIG. 7, the electrical lines 53 and 54 from the BMS control the circuit closure of the charger and the load, respectively. A temperature sensor 57 such as thermistor is also placed into the Battery 30 and is wired to the BMS 55 with an electrical connection 58.
Such a BMS has the following major intentions—                To monitor the Blocks in the Battery        To protect the battery        To estimate the battery's state of charge or instantaneous capacity        To maximize the Battery's performance by balancing the Blocks.        To communicate any important parameters of the battery to an external device or a user.        
The general management functions of such a BMS are—                1. Protection: Not allowing the battery, any block or any cell to operate outside of recommended operating parameters. Such function can be further subdivided as—                    (a) Prevent the voltage of a Block from exceeding a limit, by stopping the charging current. In Lead-Acid batteries an excess voltage would cause excess generation or hydrogen and oxygen, while in a Lithium Ion battery it can cause the cell to fail and explode, thus compromising safety.            (b) Prevent the temperature of any Cell or any Block from exceeding a limit by stopping the battery current, or requesting that it be cooled. Most Lithium Ion cells are prone to a thermal run-away if such safety mechanism is not incorporated by a BMS.            (c) Prevent the voltage of any Cell or Block from dropping below a limit by stopping the discharging current. For instance, in Lithium Ion batteries, an electrode may dissolve in the electrolyte if the Cell is allowed to discharge below a certain low voltage—around 2.3V. In case of Lead-Acid Cells, sulfation of electrodes may occur at very low battery voltages. In many cases such effects cause irreversible damage to the Cell.            (d) Prevent charging current from exceeding a limit by reducing or stopping the current. For instance, in Lead acid and Lithium Ion Cells, a higher charging current than recommended causes permanent damage to electrodes, and may result in unsafe conditions. Typically, the charge current limit is a function of Block voltage, temperature, state of charge and the previous level of current.            (e) Prevent discharging current from exceeding a certain limit by reducing or stopping the current. For instance, in Lead acid and Lithium Ion Cells, a higher discharging current than recommended causes permanent damage to electrodes, and may result in unsafe conditions. Typically, the charge current limit is a function of Block voltage, temperature, state of charge and the previous level of current.                        2. Thermal Management: Controlling the thermal actuators and devices for the Pack to maintain the temperature of the Battery, its Cells and its Blocks within a recommended range. For instance, the Pack may contain thermoelectric devices (TEC) that can add to or subtract heat from the Pack with the application of a controlled current. The Cell manufacturer's recommendation may be to run the Battery then between 15 deg C. and 35 deg C. During the operation of the battery, if the temperature falls below 15 deg C. for any block, then the TEC could be instructed to heat the pack, whereas if the temperature goes above 35 deg C., the TEC could be instructed to cool the pack. Such decisions would be taken by the BMS.        3. Balancing: Maximizing the battery's capacity by distributing or redistributing the charge among the Blocks as the battery undergoes charging and discharging.        
This invention pertains to the balancing action of the BMS. During charge and discharge of the Battery, one pushes a certain amount of charge into each Cell. If each Cell were identical in all respect, then the Battery would stay balanced at all times, but two Cells are never the same. Due to manufacturing variations, and post-manufacturing treatments, the Cells develop different characteristics, as follows, which result in their different capacity behavior.                1. Cell resistance or Equivalent Series Resistance (ESR). If the ESR of a Cell is higher compared to other Cells, it will respond with a larger polarization voltage than others in series to the response of the same charging current.        2. Capacity. Two different Blocks may not have the same electrochemical capacity, in which case, in response to the same charging current, the voltages will be different.        3. Leakage. Depending on the age of the Blocks, two different Cells in two different Blocks may have different internal leakage currents. Leakage current is responsible for self-discharge of a Cell, and therefore affecting the capacity of the Cell and in turn, of the Block. As a result, the effective charge and discharge capacities will be different, and will have different voltages in response to same charging current.        4. SOC. If the blocks started operating with different SOCs to start with, or if parasitic loads are taken off from intermediate blocks in a battery, the battery as a whole will stay unbalanced.        
Different BMS devices do the balancing in different ways. The schemes known so far include the following—                (a) Shunt Regulator Bypass: In this case a shunt power regulator is placed across each block in the BMS. During charging, when a block reaches the maximum recommended voltage, the shunt bypasses the block. Although this seems simple, the shunt regulator has to be able to carry the entire charging current in the bypass mode, which results in expensive electronics. Besides, when this happens with one or more cells, the battery charging voltage must drop keeping the current the same, thus charging the rest of the blocks. The charger needs to be able to accommodate such a voltage swing, which is not easy. Besides, if the charger is connected to a load, the load specifications may not allow this voltage swing to happen. Consequently, such a scheme is not very popular and is used only where the charging current is low (<1 A or so)        (b) Dissipation: In this case, at a pre-determined range of voltage or SOC, all the blocks that have higher voltage or SOC burn some power by trickling some current to the ground or another cell. While this remains as a popular method, it is wasteful in terms of energy. This method also creates a lot of heat in the Pack, due to which thermal management becomes difficult.        (c) Distribution: In this case, during the charging of the battery, the Blocks that have higher voltage or SOC transfer some of their capacity to the entire Battery chain or a section of it by switching regulators. While this is less wasteful than dissipative methods, it requires high current switching passives (such as inductors and capacitors), need a lot of discrete components, and reliability and cost concerns are high.        
While all the above methods are in use today, they still cannot satisfy some fundamental needs of the industry.                1. All of them still have some dissipation, and as the Cells grow older, the dissipation becomes a significant portion of the total energy transacted during charging and discharging. Besides reducing the efficiency of the product, it creates heating problems in enclosed Packs.        2. If different Blocks in a battery have Cells of different chemistries, the blocks would then have different charge and discharge termination voltages and therefore none of the schemes above would work.        3. If some Blocks have significantly higher leakage, then balancing becomes even more wasteful and may never eventually bring the Cells to an effective balance.        4. If the Blocks have different number of Cells, or have different operational history, then their effective capacities may be different, and therefore the schemes would be highly dissipative or be generally ineffective.        5. These schemes generally do not offer a good way to keep the Blocks balanced during discharging.        